Conventionally, a variety of forms of slits, which are an elongated hole formed in a base metal of a circular saw blade, have been proposed for as this kind of the disc-shaped tool. For example, Japanese Patent Application Publication No. 46-21356, Japanese Utility Model Application Publication No. 5-18010, European Patent No. 0640422A1, West Germany Patent No. 19648129A1 have disclosed this kind of the circular saw blades. It has been well known that providing with such a slit increases the critical revolution number of the circular saw blade and further filling the slit with resin makes noise and vibration, which affects the quality of a cut face adversely, unlikely to occur. The critical revolution number is a revolution number which generates buckling in a circular saw blade and generally the circular saw blade is used at a revolution number below the critical one. However, how the arrangement and the entire shape of the slit are related to the stiffness and the critical revolution number of the circular saw blade and how they affect the durability and suppression of vibration have not been clarified and therefore, it is not clear whether or not a slit having an appropriate shape is formed at an appropriate position.
The inventors analyzed in detail about how the arrangement and the entire shape of the slit formed in the disc-shaped tool affect the stiffness of the disc-shaped tool, the critical revolution number and vibration, confirmed its analysis result based on specific experiment results so as to investigate those results and then reached the present invention based on those results.
The present invention intends to solve the above-described problem and provide a disc-shaped tool which has a high stiffness and high critical revolution number and ensuring a high durability and is unlikely to generate a vibration accompanying a cutting work by specifying the arrangement position and condition of the slits.